1. Field of the Invention
The present invention relates generally to systems for controlling the transmission of light. More particularly, the present invention relates to an apparatus and method for attenuation of laser light without disturbing the relative distribution of light intensities across the beam.
2. Brief Description of the Prior Art
When making measurements of the width of laser beams, and their propagating parameters, there is a need for large amounts of attenuation of the laser light. This is because lasers concentrate a large amount of light power in small beams so that normal light detecting devices are strongly overloaded by most common laser sources.
The necessary attenuation must be accomplished without disturbing the relative distribution of the light intensities across the beam in order to make meaningful measurements of the laser beam parameters.
Further, in the case of pulse lasers in particular, it is necessary to collect samples of the beam at three (3) or more positions in space simultaneously so that the propagating parameters of a single pulse of laser light can be determined.
An unusual property of laser light, compared to more common and well-known light sources, is called its "long coherence". This means that laser light maintains a very pure wave characteristic over much longer distances than common light sources. That is, the laser light "wavetrain" maintains a uniform frequency and phase over a broad space transverse to the direction of propagation.
These pure waves, if combined together from the various reflections of an optical system, cause what are called "interference" effects. As generally defined, interference of waves is the process whereby two or more waves of the same frequency or wavelength combine to form a wave whose amplitude is the sum of the amplitudes of the interfering waves. This translates into regular increases and decreases of the light intensity across the laser beam. Pure waves in an optical system may travel over different path lengths and may have either the same wave-timing or opposite wave-timing. Thus, for two waves having the same wave-timing, their wave peaks add together and the light intensity increases. For two waves having opposite wave-timing, the peaks of the first wave coincide with the troughs of the second wave and the waves cancel each other and the light intensity decreases.
The commonly available optical attenuators of the prior art were originally developed for use with ordinary or "impure" light. Such light has a very short coherence length, typically only small fractions of a millimeter. The ordinary impure light waves can only cause interference effects over very short distances. Because of this, reflections in ordinary optical systems do not cause perceptible interference effects. Most optical system reflections are separated by much greater distances than a fraction of a millimeter; the reflections cause some reduction of the ordinary light passing through an ordinary light optical system but do not cause local fluctuations of the light.
The need for substantial attentuation of laser beams for measurement purposes requires a number of attenuators in the optical system. Also, these attenuators should be variable in order to accommodate a wide range of laser powers and beam sizes. It is also necessary that none of these attenuators cause interference effects or other distortions of the output beam.
In addition to interference effects, ordinary optical attenuators may be heated by the laser beam itself causing local optical distortion of the beam or even destruction of the attenuator itself.
The prior art recognizes some of the difficulties outlined above but a complete solution has not yet been presented. The costs of some of the partial solutions made available to this time are unnecessarily high or distort the beam in some manner.
U.S. Pat. No. 4,925,273 issued to Maisenbacher et al., recognizes the need to avoid interference effects in a single attenuator and does so by tilting the outgoing surface of the attenuator with respect to the incoming surface of the attenuator so that the light reflected at the outgoing surface is reflected away from the optical path of the incoming beam. Maisenbacher also teaches to leave the incoming surface of the attenuator uncoated so that a strong interference occurs between the orthogonal attenuator surface and the output mirror of the laser itself. However, the minimum degree of tilting necessary to achieve a desired result and avoid other undesirable effects is not addressed. Further, Maisenbacher does not address the more important need to control the interference effects between successive attenuators positioned in series.
The strongest interference effects in an optical system which is composed of successive attenuators occur between the outward facing surfaces of adjacent optical parts. The interference effect within a neutral density (N.D.) attenuator is reduced by the internal attenuation.
U.S. Pat. No. 3,942,899, issued to Longhenry discloses a calibrating device for a photometering analyzer for measuring light scattering characteristics of bacterial colony samples. The calibrating devices include a wedged shaped piece of neutral density glass to obtain continuously variable attenuation. The Longhenry patent does not consider that the wedged attenuator introduces a uniform change of attenuation across the beam along the direction where the wedge thickness is changing, thus changing its intensity distribution.
U.S. Pat. No. 3,538,335, issued to Tartanjan discloses the use of a neutral density filter comprising two variable attenuators, oriented in opposed fashion, which are adapted to cancel the change of attenuation across a light beam transmitted therethrough. In a first embodiment, the attenuators are optical discs which vary linearly in optical density with angular displacement. The discs are counter rotated with respect to each other to achieve a neutral density filter. Tartanian also suggest to substitute in place of one of the discs a single neutral density wedge which is fixed in position and varying in neutral density. In a second embodiment, the filter comprises a pair of opposed facing film segments, each of which vary in opacity from one extreme to another and are oriented with respect to each other such that the attenuation is uniform through the filter. Tartanian does not address the need for avoiding interference effects between the successive attenuators of the filter, nor is there any teaching or suggestion for selecting a particular wedge angle to overcome interference effects when a fixed wedge attenuator is proposed. Also the cost advantage of fabricating the variable attenuators from neutral density glass is neither recognized nor addressed in this patent.
When measuring high power lasers the general need to avoid absorption in the first attenuation steps has been recognized by many workers in the field. For example, strongly-wedged single beam-splitter product, Model LBS-100, has been offered by Spiricon Corporation of Logan, Utah, for this application. However, this single splitter changes the direction of the beam substantially. This means that the beam can not be used for its intended purpose while being measured, unless the entire optical train is rearranged. The strong wedge also alters the size of the beam in one axis.
From U.S. Pat. Nos. 5,064,284 and 5,078,491, both issued to Johnson, there is disclosed an apparatus for laser mode "quality" measurement which is suitable for continuous wave lasers. This apparatus is not useful for measuring single laser pulses, since the beam size data is not collected simultaneously at multiple positions in space. There is no mention of the need to avoid interference effects in and between the optical elements.
To solve the various subtle problems and requirements for attenuating laser light for beam measurement purposes, a new and useful system of special optical attenuators, and their appropriate combination, is here proposed.
The system to be described herein provides both a non-distorting wide-range attenuation method and the simultaneous measurement of the beam at multiple positions using a non-distorting attenuation method.